ErosionErosion

►The removal of rock material The removal of rock material (weathered rock, boulders, gravel, (weathered rock, boulders, gravel, sand, silt and clay)sand, silt and clay)

►Erosion suggests transport, but Erosion suggests transport, but transport is not necessarily erosion.transport is not necessarily erosion.

TransportTransport

►Weathered rock and sediments and Weathered rock and sediments and dissolved minerals are moved from the dissolved minerals are moved from the source area to the depositional area.source area to the depositional area.

►Transport agentsTransport agents Running WaterRunning Water WindWind IceIce

DepositionDeposition

►Deposition occurs when the energy of Deposition occurs when the energy of the sediment transport agent the sediment transport agent decreases.decreases.

►Deposition occurs when the sediment Deposition occurs when the sediment can no longer be moved.can no longer be moved.

LithificationLithification

• Lithification involves compaction and Lithification involves compaction and cementation that convert sediment cementation that convert sediment into sedimentary rock. into sedimentary rock.

• Silica and calcium carbonate are the Silica and calcium carbonate are the most common chemical cements, but most common chemical cements, but iron oxide and iron hydroxide cements iron oxide and iron hydroxide cements are important in some rocks.are important in some rocks.

RiversRivers

►Rills to Rivers (channel flow)Rills to Rivers (channel flow)►Sources: Sources:

sheet flow from rain and snowsheet flow from rain and snow groundwatergroundwater soil moisturesoil moisture

►Topographic High to Low (eventually to Topographic High to Low (eventually to sea level—base level)sea level—base level)

►Drainage Basin (all sheet flow and Drainage Basin (all sheet flow and channel flow ends up in one major channel flow ends up in one major channel flow)channel flow)

Fig. 12-3, p. 274

Water Cycle

RiversRivers

►Rivers erode vertically and laterallyRivers erode vertically and laterally►Rivers transport sediments and Rivers transport sediments and

dissolved mineralsdissolved minerals►Rivers deposit sediments Rivers deposit sediments

Figure 1a, p. 273

Hoover Dam and Lake Mead

Erosion or deposition?

Fig. 12-1, p. 270

Note v-shaped valley shape

Note steep gradient (much downcutting, cannot meander)

Erosion or deposition?

Fig. 12-CO, p. 268

What happens when Niagara slips?

Niagara Falls!

http://www.niagaraparks.com/nfgg/geology.php

http://www.lockport-ny.com/Museum/rocks.htm

Horseshoe Falls

How do rivers erode?How do rivers erode?

►Cutting power of rivers is recognized Cutting power of rivers is recognized over time and during periods of over time and during periods of excessive flow (e.g., flood stage)excessive flow (e.g., flood stage)

►Steeper gradients increase Steeper gradients increase downcuttingdowncutting

►Shallow gradients decrease Shallow gradients decrease downcutting but increase lateral flow.downcutting but increase lateral flow.

Fig. 12-4, p. 274

Rapid stream velocity, bed load is coarse, erosion is downward.

Slow stream velocity, bed load is fine, erosion is lateral, stream meanders

Stream Gradient

head

mouth

Fig. 12-5b, p. 275

Stream FeaturesStream Features

► LoadLoad Sediment load (boulders, gravel, sand), also called bed Sediment load (boulders, gravel, sand), also called bed

loadload Suspended load (silt and clay) high energy events can Suspended load (silt and clay) high energy events can

carry sand and gravelcarry sand and gravel Dissolved load (dissolved minerals)Dissolved load (dissolved minerals)

► Primary and Secondary ChannelPrimary and Secondary Channel► Flood PlainFlood Plain► Cut Bank vs. Point BarCut Bank vs. Point Bar► Meanders and Ox-Bow LakesMeanders and Ox-Bow Lakes► DeltaDelta► Alluvial Fan (intermittent stream)Alluvial Fan (intermittent stream)

Stream LoadStream Load

http://www.indiana.edu/~g103/G103/week6/wk6.html

Fig. 12-7a, p. 278

Braided Stream, New Zealand

Can be caused by additional influx of sediments or a change in topography causing a lowering of the stream gradient

Fig. 12-7a, p. 278Braided stream, Chester, California

Fig. 12-11, p. 281

Primary and Secondary Channels of a StreamPrimary Channel

Secondary Channel

During a flood, the secondary channel begins to fill

Fig. 12-12a, p. 281

Fig. 12-12b, p. 281

Fig. 12-11b, p. 281

Fig. 12-11c, p. 281

Fig. 12-11d, p. 281

Fig. 12-9, p. 279

Fig. 12-9a, p. 279

Fig. 12-9b, p. 279

Fig. 12-6a, p. 277

Sediment load added by erosion at the river bank

Cut Bank

Fig. 12-9c, p. 279

Fig. 12-11a, p. 281

Cut Bank Point Bar

Crossbedding

Fig. 12-8, p. 278

Meandering Stream, low stream gradient, Laxa River, Iceland

Note preliminary formation of an ox-bow lake

Fig. 12-10, p. 280

Fig. 12-10a, p. 280

Fig. 12-10b, p. 280

Fig. 12-10c, p. 280

Fig. 12-10d, p. 280

Fig. 12-23, p. 291Incised meanders—area must have undergone some uplift

Fig. 12-13a, p. 282

Delta

Velocity of the stream is reduced at the mouth of the river.

The sediment and suspended load begins to settle out

Sorting: coarser grains nearer the mouth, finer grains further away

Fig. 12-13b, p. 282

Topset Beds

Foreset Beds

Bottomset Beds

Delta Deposition Pattern

Fig. 12-13c, p. 282

Mississippi River Delta— birds foot delta

Fig. 12-14, p. 283

Alluvial Fans (much coarser-grained than deltas)

Form when heavy rainstorms occur in sparsely-vegetated semi-arid regions.

Fig. 12-14a, p. 283

Fig. 12-14b, p. 283

Fig. 12-21, p. 290

Idealized Stages in the Development of a Stream.

Fig. 12-21a, p. 290

Younger, steeper gradient

Downcutting erosion

Fig. 12-21b, p. 290

Intermediate gradient

Fig. 12-21c, p. 290

Low gradient, mature stream approaching base level

Fig. 12-22, p. 291

Eventually, downcutting will lead to the formation of terraces (which are erosional remnants of floodplains.

Fig. 12-22a, p. 291

Fig. 12-22b, p. 291

Fig. 12-22c, p. 291

Fig. 12-22d, p. 291

Fig. 12-15, p. 285

Drainage Basin

Fig. 12-15a, p. 285Small scale drainage basins follow topography

Divide

Fig. 12-15b, p. 285

Wabash River Drainage Basin

Fig. 12-16, p. 286

Stream Drainage Patterns

Fig. 12-16a, p. 286

Dendritic

Fig. 12-16b, p. 286

Rectangular—controlled by solution joints

Fig. 12-16c, p. 286

Trellis

Fig. 12-16d, p. 286

Radial

Fig. 12-16e, p. 286

Deranged—swampy regions

Fig. 12-17, p. 288

Fig. 12-17a, p. 288

Fig. 12-17b, p. 288

Fig. 12-17c, p. 288

Fig. 12-18, p. 289

Fig. 12-18a, p. 289

Fig. 12-18b, p. 289

Fig. 12-19, p. 289

Fig. 12-19a, p. 289

Fig. 12-19b, p. 289

Fig. 12-20, p. 289

Headward erosion in a valley and stream piracy.

Arrgh!

Fig. 12-20a, p. 289

Fig. 12-20b, p. 289

CHAPTER OUTLINE

• Introduction

• The Hydrologic Cycle

• Running Water

• GEO-FOCUS 12.1: Dams, Reservoirs, and Hydroelectric Power

• How Running Water Erodes and Transports Sediment

• Deposition by Running Water

• Drainage Basins and Drainage Patterns

• CULTURAL CONNECTIONS: Flood Stories from Around the World

• Base Level

• Graded Streams

• Valley Evolution

• Geo-Recap

CHAPTER OBJECTIVES

1 Running water, one part of the hydrologic cycle, does considerable geologic work. Water is continuously cycled from the oceans to land and back to the oceans.

2 Gradient measures how steep a stream is. Discharge measures the volume of water that passes a given point per unit of time. Discharge, along with velocity, usually increases downstream.

3 Running water transports large quantities of sediment and deposits sediment in or adjacent to braided and meandering rivers.

4 Flooding is a natural part of stream activity that takes place when a channel receives more water than it can handle.

5 Alluvial fans (on land) and deltas (in a standing body of water) are deposited when a stream’s capacity to transport sediment decreases.

6 Rivers and streams continuously adjust to changes. Base level is the elevation below which a stream cannot erode. Waterfalls and lakes are temporary base levels, and the sea is ultimate base level.

CHAPTER OBJECTIVES

7 The concept of a graded stream is an ideal, although many rivers and streams approach the graded condition.

8 Most valleys form and change in response to erosion by running water coupled with other geologic processes such as mass wasting.

Fig. 12-2, p. 270

Available Fresh Water

Fig. 12-5, p. 275

Figure 1b, p. 273

Fig. 12-5a, p. 275

CHAPTER SUMMARY

• Water continuously evaporates from the oceans, rises as water vapor, condenses, and falls as precipitation. About 20% of all precipitation falls on land and eventually returns to the oceans, mostly by surface runoff.

• A channel’s gradient varies from steep in its upper reaches to gentle in its lower reaches.

• Flow velocity and discharge are related, so a change in one results in a change in the other. Velocity and discharge increase downstream in most rivers and streams.

• Running water erodes by hydraulic action, abrasion, and dissolution of soluble rocks.

• The larger particles transported by running water move as bed load, whereas the smallest particles move as suspended load. Rivers and streams also transport a dissolved load of materials in solution.

• Braided streams have complex, multiple, intertwining channels. Their deposits consist mostly of sand and gravel.

CHAPTER SUMMARY

• Meandering streams have a single, sinuous channel in which point bars of sand or gravel are deposited. Cutoff meanders known as oxbow lakes eventually fill with fine-grained sediments and organic matter.

• Floodplain deposits might consist of a succession of point bars deposited by a migrating channel, or mud deposited by water carried into the floodplain during floods.

• Deltas form where a river or stream enters a standing body of water and deposits its sediment load. Small deltas in lakes commonly have a three-part division of bottomset, foreset, and topset beds, but marine deltas are larger, more complex, and more important economically.

• In arid and semiarid regions where a river or stream flows from a mountain canyon onto adjacent lowlands, a deposit known as an alluvial fan accumulates. Alluvial fans consist of stream-deposited sand and gravel and/or mudflow deposits.

• Rivers and streams along with their tributaries carry runoff from areas known as drainage basins, which are separated by divides.

CHAPTER SUMMARY

• Sea level is ultimate base level, the lowest level to which rivers and streams can erode. Local or temporary base levels are lakes, other rivers or streams, or particularly resistant rocks.

• Rivers and streams tend to eliminate irregularities in their channels so that they develop a smooth, concave profile of equilibrium. These so-called graded streams approach this ideal condition only temporarily.

• Valleys develop and evolve by several processes, including downcutting, lateral erosion, headward erosion, stream piracy, and mass wasting.

• The formation of a floodplain, followed by renewed downcutting by a stream, leaves remnants of the older floodplain at higher levels known as stream terraces.

Fig. 12-6b, p. 277

Mass wasting

Sediment added to stream by mechanical weathering and erosion

Fig. 12-6c, p. 277

Pothole formed by cutting power of falling water and abrasion from rock inside the pothole.

Fig. 12-6d, p. 277

Note roundness of the boulders inside the pothole.

Stream DepositsStream Deposits

► LeveeLevee► Flood plain depositsFlood plain deposits► Braided streams (sediment load is greater Braided streams (sediment load is greater

than velocity can move)than velocity can move)► Deltas (velocity decreases as stream enters Deltas (velocity decreases as stream enters

larger body of water-not channel flow, larger body of water-not channel flow, allowing for deposition)allowing for deposition)

► Alluvial fans (“semiarid delta”)Alluvial fans (“semiarid delta”)

Stream GradientStream Gradient

►Slope of the channel as it flows downhill.Slope of the channel as it flows downhill.►Steeper gradient in upper reaches, near Steeper gradient in upper reaches, near

headwaters, headwaters, ►Stream gradient gets progressively Stream gradient gets progressively

smaller towards base level.smaller towards base level.►Steep gradient streams are generally Steep gradient streams are generally

straight and cut downward.straight and cut downward.►Shallow gradient streams meander and Shallow gradient streams meander and

generally cut sideways (laterally).generally cut sideways (laterally).